Artificial illumination



May 14,1940 w. ELENBAAS ARTIFICIAL ILLUMINATION Fi led Nov. 15, 1957 INVENTOR WiHem EIenlo as BY 5 AT ORNEY Patented May 14, 1940 PATENT, OFFICE ARTIFICIAL ILLUMINATION Willem Elenbaas, Eindhoven, Netherlands, assignor to General Electric Company, a corporation of New York Application November 13, 1937, Serial No. 174,456

In Germany November 20, 1936 2 Claims.

The present invention relates to a novel source of artificial illumination which will closely approximate daylight.

V A particular object of the invention is to provide a source of white light. Another object of the invention is to provide a light source of high efiiciency. A further object of the invention is to provide a light source which can be connected to'a source of fixed electrical potential without 10 the use of auxiliary apparatus. Still other obv jects and advantages of the invention will appear from the following detailed specification or from an inspection of the accompanying drawing.

The-invention consists in the new and novel ll combination of elements hereinafter set forth and claimed.

A source of so-called "white light whichwould closely approximate daylight and thus make all objects, regardless of color, have the same appearance as in daylight has long been sought, since such a light source would be useful in many places. Various efforts have been made from time to time to produce such a source, as by combining two different light sources of more or less I complementary color characteristics, but all of these have left something to be desired, either as a result of imperfect color mixture, or because of the bulk or complicated nature of the source,

or both. The structures proposed; have likewise i been relatively expensive.

By the present invention I have solved this problem by producing a novel lamp havingthree different, but mutually cooperating, light sources enclosed within a single bulb, the whole unit be- 8 ing so arranged that it can be screwed into a conventional socket to replace an. incandescent lamp without the'need of any auxiliary apparatus or without any change in the circuit. The diflerent light sources within this lamp combine to pro- 49 duce a substantially white light.

This novel lamp is characterized by the novel combination within'a single lamp of a gaseous discharge device, an incandescent filament which also serves as the ballast for this discharge dell vice, and a luminescent material which is subject to. the rays emitted by both the discharge device-and the filament. In general this lumi-,

- nescent material is coated on or mixed in the material of the bulb which houses both the filasoment and the discharge device. Such a device is obviously self-contained, needing no auxiliary apparatus, and can be manufactured simply and without excesive cost. The luminescent material, which serves as the third light source, does I not make the lamp any more complicated, since the bulb on which it is placed is required to protect the discharge device and to enclose the filament. The space within this bulb is either evacuated or filled with a gas which is inert with respect to both the filament and the luminescent coating.- The discharge device used may be of any desired type, but is preferably one emitting some ultraviolet rays. In practice I prefer to use a mercury vapor arc lamp of the'type disclosed in the application of B01 et al., Serial No. 46,952, filed October 26, 1935, due to the fact that these lamps, which are now on the market, are not only compact, so that they readily lend themselves to my novel combination, but are also extremely eflici'ent and also emit a large amount of ultraviolet radiations which excite the luminescent material.

In this novel combination the discharge device plays its usual function of producing visible light, and also excites the'luminescent material whereby 17115136136! provides more visible radiations, which are preferably complementary to those of the discharge device. At the same time the incandescent filament plays its usual role of emitting visible light, plus a new and wholly unexpected function of increasing the efliciency of the luminescent material in its response to the rays from the discharge device. Thus I have discovered. that the infra-red rays emitted by the filament in my novel combination upon striking the'luminescent material accelerate the return of the excited centers of the luminescent material to their normal state, and thus cause an increase in the efficiency of the luminescent material. v

With the three light sources thus available within my novel lamp I have found that light of any desired color, including white, is produced by a suitable choice of the individual sources and of the relative intensities thereof.

For the purpose of illustrating my invention I have shown in Fig. 1 of the accompanying drawing a graph illustrating the color values of the various light sources in a preferred embodiment, and in Fig. 2 I have shown, in partsection, a preferred form of my novel light source.

In order to give a substantially white light, the discharge tube, the filament and the luminescent material are preferably selected in such a way that the color point of the combined light is located in the white field of the color triangle. As known, each light color corresponds to a certain point in the color triangle. Fig. 1 in the drawing shows this color triangle in accordance with the system determined by the International Commission on Illumination in 1931. In the center of this triangle the point is located which indicates the white light. In the vicinity of this point there are other points which correspond to color impressions which difier but slightly from white. The dotted ellipse A in Fig. 1 encloses points which indicate light colors which may be indicated as being white without being too inaccurate. The field enclosed by the dotted ellipse will be called the white field. The position of this ellipse in the color triangle will be defined as follows. The ends of the long axis of the ellipse lie on the curve B which shows the colors of the light emitted by a black body at various temperatures. The points C and D (the ends of the long axis) correspond to the radiation of the black body, at 3900 and 8000 Kelvin. The ends of the short axis of 'the ellipse are characterized by the degree of saturation of thelight color indicated by these points. The degree of saturation of the points E and G is 0.20 and minus 0.25;

If the three components ofthe lamp, according to the invention, are selectedin such a way that the color points of these components lie in the color triangle, so that the triangle formed by these pointsencloses the white field, then it is possible by a suitable selection of the intensity of the light emitted by each light source, to place the color point of the combined light at any desired position of the white field. It is, however,

not necessary that the triangle formed by the color points of the three components encloses the entire white field. Even when only'a part of the white field is enclosed, it is possible to attain many color tones which closely approach white.

The color point of the light emitted by the discharge tube depends, of course, on the type of the filling and frequently on the current and on v the pressure of the filling. The color point of the light emitted by the filament depends on the temperature of the wire, which can be adjusted by giving the wire suitable dimensions. The color point of the light emitted by the luminescent bulb is determined by the type of the luminescent material.

With reference to the lamp shown byway of example in Fig. 2, I is a discharge tube' consisting of a quartz tube with aninternal diameter of 4 mm. and an external. diameter of 7 mm. The two electrodes 2 and 3 which are located in this discharge tube are heated by the discharge and are preferably covered by means of a material which promotes the emission of electrons, such as an alkaline earth oxide. The spacing of these electrodes in the device shown is 8 mm. The discharge tube contains a quantity of argon which has a pressure of, for instance, 30 mm. at room temperature. In' addition, a quantity of mercury is located in the tube which evaporates during operation to produce a very high mercury vapor pressure of approximately 15 atmospheres.

The discharge tube I is enclosed in a' completely closed bulb' 4 which consists of ordinary glass. The bulb has the shapeof an ordinary incandescent lamp bulb. The discharge tube I is fastened to the stem tube 6 of the bulb 4 by means of the supporting wires 5 which at the 1 same time constitute the current supply wire for the electrode 2. The bulb is provided with a screw base 7. The bulb 4 is preferably filled with nitrogen which at room temperature has a pressure of about 50 cm.

the discharge current flows through it so that it servesv simultaneously as the series ballasting resistance.

Since the voltage of the high-pressure vapor discharge tube l increases considerably after the 5 ignition whereby the dangeris created that the filament 8 may be overloaded during the heating period of the discharge tube, it is advantageous to provide means within the bulb for preventing this over-load of the filament. Thus it is desir-. 10 able to connect a resistance in. series with the filament used in normal operation; during the ignition of the discharge tube this resistance lies in series with the discharge tube and the filament, keeping the current within desired limits. 15

After the burning voltage of the discharge tube has .increased to a certain value, this additional. resistance is no longer necessary, and hence it can be removed or reduced in any desired manner', as by means of a "bimetallic switch which is operated by the heat produced in the discharge tube, or as shown, by means of a resistance It) i silicate and tragacanth, both in the powdered form, and then adding water to form a homogeneous mass. Small rods are then pressed from this mass while at the same time small blocks of graphite are preferably pressed into the ends of the rod which serve as the contact elements of the resistance. heated in a reducing gas atmosphere in such a way that the material is sinte'red. By selecting the ratios of the mixed components and by suitably controlling the temperature at which the sintering takes place, the ratio of the resistance' 4 values at room temperature and at the operating temperature is regulated.

In parallel with the resistance It! there is connected a resistance II which consists of tungsten and which heats the resistance It when acurrent passes therethrough. When first placed in operation, practically the entire current flows through the wire II; the heat developed in this wire then raises the resistance It) to a higher temperature whereby the electrical resistanee' is greatly reduced.

The inside of the bulb 4 is coated with a luminescent layer l2. This layer is excited by the rays emitted by the discharge tube l, and emits visible light, this emission being increased by the 60 effects of the infra-red rays from the filament 8, as pointed out hereinbefore. The luminescent layer in the preferred embodiment consists of a; mixture of zinc sulphide activated by silver and of zinc cadmium sulphide activated by copperx' The ratio of the two components of this mixture is selected in such a way that the light emitted by this luminescent mixture has a color point which is indicated in Fig. 1 by H. a

The lamp is arranged to be connected to-a network of 220 volts at a commercial frequency. The current which flows through the tube l afterit has reached equilibrium is 0.5 ampere. The power consumptionof the discharge tube is in that case 40 watts while the mercury vapor pres- L i The small rods are dried'and 40 sure is approximately 15 atmospheres. The light emitted by this mercury vapor discharge tube has a color point which in Fig. 1 has been indicated by K.

The filament 8 is dimensioned in such a way that it attains a temperature 01 2800 Kelvin during operation and in that case emits 770 international lumens of visible light. The color point of this light is indicated in Fig. 1 by L.

The color point of the combined light of the mercury vapor discharge tube, of the filament and or the fluorescent material is indicated in Fig. 1 by M. As shown, this point represents a practically white light.

The novel lamp thus produced not only gives an exceptionally good white light, however, but

is also extremely efficient, not only due to the utilization of the ultraviolet radiations and the ballasting energy of the discharge device to produce light complementary thereto, but also due to the increased efliciency of the luminescent coatlight.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An electric lamp comprising, in combination, an electric gaseous discharge device which emits ultraviolet and visible radiations when a discharge is passed therethrough, a filament which is adapted to be operated at incandescence to supplement the light from said discharge device, and a luminescent bulb which encloses both said discharge device and said filament, said luminescent bulb having a response to the ultraviolet radiations a mercury vapor arc 'device adapted to emit both visible light and ultraviolet radiations, a filament connected in series with said device and adapted to be raised to incandescence by the current there- ,through, and a light transmitting bulb enclosing both said device and said filament, said bulb having a coating' thereon of a luminescent material comprising a mixture of zinc sulphide activated by silver and zinc cadmium sulphide activated by copper, whereby said luminescent material is irradiated by both the ultra-violet from the arc device and the ultraviolet and the infra-red from the filament to produce visible radiations which supplement the visible light from said device and said filament to produce a substantially white WILLEM ELENBAAS.

,which is increased by the effect of infra-red rays 

